This invention relates to a combination valve plug and valve orifice used to control flow under throttling conditions in order to regulate the level, temperature or pressure in process control applications. Conventional trim of this type usually consists of a lathe-turned plug of generally parobolic shape axially displaced in a cylindrical orifice. The annulus formed between the outer periphery of the plug and the inside diameter of the orifice provides the desired flow area at a given lift position. The relation between such a flow area at any given lift to the flow area at maximum lift determines the flow characteristic of such a trim. The combination of effective flow area and velocity headloss then defines the flow capacity usually expressed in Cv, where 1 Cv is the flow of 1 US gpm of water passing through a restriction under a pressure drop of 1 psi. From the Darcey equation: ##EQU1## wherein C is a contraction coefficient and K is the velocity headloss coefficient.
In conventional trim systems the flow characteristic can only be determined through variations in flow area (requiring a precision machining of the valve plug), since the velocity headloss coefficient, i.e. the fluid resistance, is for all practical purposes constant (usually K .apprxeq. 0.7). Furthermore, the relatively smooth flow path between a parablolic plug and an orifice can lead to pressure recovery and therefore cavitation on liquids. In addition, most process control systems require that the pressure drop across the valve should rise inverse proportional to the square of the decrease in flow rate (i.e. at 25% flow the pressure drop can be 16 times higher than that at 100% flow), due to pump droop and line resistance in series with the valve requiring a flow characteristic commonly called "equal percentage" to compensate for the non-linearity of pressure drop. Again, the accuracy of such a characteristic depends on the precision machining of a complex curvature on the valve plug. The high differential pressure .DELTA. p at low flow rates produces excessive fluid velocities (due to the relatively low velocity headloss coefficient K. since V = f (.DELTA.p /K).sup.1/2. High velocities cause erosion of the trim with liquids and substantial aerodynamic noise with gases (Sound Pressure Level SPL = f (V).sup.8 ).